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Re: O_DIRECT for WAL writes

From: Bruno Wolff III <bruno(at)wolff(dot)to>
To: Mary Edie Meredith <maryedie(at)osdl(dot)org>
Cc: Neil Conway <neilc(at)samurai(dot)com>,ITAGAKI Takahiro <itagaki(dot)takahiro(at)lab(dot)ntt(dot)co(dot)jp>,pgsql-patches(at)postgresql(dot)org
Subject: Re: O_DIRECT for WAL writes
Date: 2005-06-03 19:24:51
Message-ID: 20050603192451.GA25970@wolff.to (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
On Fri, Jun 03, 2005 at 09:43:13 -0700,
  Mary Edie Meredith <maryedie(at)osdl(dot)org> wrote:
> 
> Looking at this from another angle, is there really any way that you can
> say a write is truly guaranteed in the event of a failure?  I think in
> the end to be safe, you cannot.  That's why (and I'm not telling you
> anything new) there is no substitute for backups and log archiving for
> databases. Databases must be able to recognize the last _good
> transaction logged and roll forward to that from the backup (including
> detecting partial writes to the log).  I'm sure the PostgreSQL community
> has worked hard to do the equivalent of that within the PostgreSQL
> architecture. 

Some assumptions are made about what order blocks are written to the disk.
If these assumptions are not true, you may not be able to recover using
the WAL log and have to resort to falling back to your last consistant
snapshot.

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